<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    #block {
      width: 400px;
      height: 400px;
      background-color: #69c;
      margin: 0 auto;
      -webkit-transition: background-color 3s;
    }
    #block:hover {
      background-color: green;
    }
  </style>
</head>
<body>
  <div id="block"></div>
</body>
</html>